In the art of deposition of films of material onto a surface of a substrate, there are many known techniques, including vacuum evaporation deposition, ion plating, ion- and plasma-assisted sputtering or chemical vapor deposition ("CVD"), and the more modern ICB approach. ICB deposition is an ion-assisted technique in which the material to be deposited on a substrate is heated in a crucible and its vapor ejected through a small nozzle into a vacuum region. The vapor forms loosely-held atomic clusters, each cluster comprising 100 to 2000 atoms of the material. Some of the ejected vaporized atomic material is ionized by electron bombardment and the atoms are accelerated toward the substrate disposed in the vacuum region. The ionized material, together with the neutral (i.e., non-ionized) component of the vapor, arrive at the substrate surface for deposition thereon. ICB deposition offers the ability to precisely control the deposited film structure by applying kinetic energy to the vapor clusters during film deposition. Kinetic energy control is achieved by varying the acceleration voltage and the electron current for ionization.
In most of the known ion- and plasma-assisted deposition techniques, the individual atoms of the material to be deposited on the substrate generally impact the substrate surface with excessive kinetic energy, producing a relatively high number of defects in the substrate and/or the deposited film. With ICB deposition, a more useful lateral energy is obtained as the clusters impact the substrate and the atoms break off, without damaging the film and substrate. Due to the effects of ionized cluster bombardment, ICB deposition produces films with high density, strong adhesion, a low impurity level, and a smooth surface. Also, film properties usually associated with relatively high substrate temperatures in conventional vacuum depositions can be obtained at lower substrate temperatures in the ICB technique. This results in a distinct advantage in semiconductor device fabrication. Examples of the ICB method of deposition of films of material onto a substrate are found in U.S. Pat. Nos. 4152478, 4217855, 5350607 and 5380683, all of which are hereby incorporated by reference.
Recent advances have been made in the field of eleclrochromics. As used herein, "electrochromics" refers to color-active glass materials that change their color and other optical properties (i.e., transmittance and reflectance) in response to an applied electric field. A number of different materials have been investigated for use as electrochromics. One of these materials is tungsten oxide, WO.sub.3. See, for example, Demiryont, "Electrochromics and Potential Automotive Applications", Optical Thin Films III: New Developments, SPIE Vol. 1323, (1990), pp. 171-187, which is hereby incorporated by reference. Tungsten oxide has desirable optical properties, including a relatively high (e.g., 2.1) index of refraction.